The Mechanism Of Direct Preparation Of Carbon Nanotubes By Fe/K-catalyzed Pyrolysis Of Coal

Carbon nanotube is an allotrope of carbon,and is a cylindrical seamless hollow tube formed by one or more graphene sheets curled.The unique one-dimensional structure makes carbon nanotubes have excellent mechanical,electrical,thermal and adsorption characteristics,and are widely used in composite materials,electronic materials and energy materials.The market demand for carbon nanotubes is growing rapidly,but due to the high requirements for raw material purity of carbon nanotubes preparation technology at this stage,the cost of carbon nanotubes preparation is relatively high,which seriously limits the practical application scope and scale of carbon nanotubes.At present,the society urgently needs the new technology for low cost and macro carbon nanotubes production.In this thesis,the carbon nanotubes with good morphology were prepared by the new method of Fe/K catalyzed pyrolysis of coal for the first time.The following four key problems in this method have not been solved yet:(1)the effect of pyrolysis conditions on the method;(2)the basic physical and chemical characteristics of carbon nanotubes prepared by Fe/K catalyzed coal pyrolysis and the growth mechanism of carbon nanotubes in this process;(3)the changes of primary minerals in coal during pyrolysis and its mechanism for the growth of carbon nanotubes;(4)the influence mechanism of Fe and K components in Fe/K catalyst on the formation process of carbon nanotubes,as well as the carbon source structure required for the growth of carbon nanotubes and its transformation process.Based on this,the process and mechanism of the direct preparation of carbon nanotubes by Fe/K catalyzed pyrolysis of bitumite coal rich in siderite were systematically studied in this thesis.The effects of pyrolysis conditions,primary minerals in coal and Fe,K components in catalyst on the morphology,yield,distribution and growth process of carbon nanotubes were studied.The growth mechanism of carbon nanotubes directly prepared by Fe/K catalytic pyrolysis of coal was established.The main results and conclusions are as follows.1.The effects of the pyrolysis conditions of raw coal on the gas precipitation process and output of H₂,CO₂,CO,CH₄,C₂H₆,C₂H₄ and C₃H₈,and the distribution of gaseous,liquid and solid products of coal pyrolysis under different pyrolysis conditions were as follows:When the pyrolysis temperature of raw coal is increased within 600-900?,the output of H₂,CO₂,CO,and CH₄ gradually increased,but the increase rate gradually decreased,the semi-coke production rate gradually decreased,and the yield of water,gas and tar increased;within 0-30 min,with the prolonged of constant temperature,the growth rate of H₂,CO₂,CO,and CH₄ gradually decreased,while the output of C₂H₆,C₂H₄ and C₃H₈ basically remains unchanged.The semi-coke production rate slightly decreased,the gas production rate slightly increases,the water production rate gradually increases and the tar production rate gradually decreases;in the range of 5-20?/min,with the accelerated of heating rate,the accumulative output of H₂,CO₂,CO,CH₄,C₂H₆,C₂H₄ and C₃H₈ showed a trend of increasing first and then decreasing,the semi-coke production rate was basically unchanged,the gas production rate increased first and then decreased,the tar production rate gradually decreased,and the water production rate gradually increased;in the range of 0-100 wt%,with the increased of KOH addition,the output of H₂,CO₂ and CO increased first and then decreased,while that of CH₄,C₂H₆,C₂H₄ and C₃H₈ gradually decreased,the yield of semi-coke and water showed a decreasing trend,the gas production increased first and then decreased,and the tar production decreased first and then significantly increased.2.The effects of pyrolysis conditions on the morphology,reactivity and carbon nanotubes content of the pyrolysis products of coal were as follows:when the temperature was increased in 600-900?,the surface of semi-coke became smoother,but the particle size and reaction activity were reduced,and the weight loss rate of semi-coke DTG can reach the maximum at a higher temperature;in 0-30 min,the pore structure on the surface of semi-coke disappeared gradually,and the temperature required for the maximum weight loss rate of semi-coke increased gradually;in the heating rate of 5-20?/min,the morphology of semi-coke was similar and there was no obvious pore structure on the surface,and the maximum weight loss rate of DTG was close to the corresponding temperature;carbon nanotubes appeared in the semi-coke obtained from pyrolysis of raw coal under different KOH addition amounts,but the carbon nanotubes had different morphologies.The content of carbon nanotubes in each sample was 1.71 wt%,8.97 wt%,and 12.02 wt%when KOH was added at 30.0 wt%,50.0 wt%,and100.0 wt%.3.When the addition amount of KOH was 50 wt%,the influence of different pyrolysis conditions on the content of carbon nanotubes in KOH catalyzed coal pyrolysis products was as follows:pyrolysis temperature>constant temperature time>heating rate;The pyrolysis temperature of 900?,the heating rate of15?/min,and the constant time of 60 min were the optimal horizontal combination of carbon nanotubes prepared by KOH catalytic coal pyrolysis,and the corresponding content of carbon nanotubes in the sample was 10.01 wt%.The order of the ability of various factors to change the composition and output of coal pyrolysis gas was:KOH content>pyrolysis temperature>heating rate>constant temperature time,and the ability to change the gas,liquid and solid product yield of coal pyrolysis was:KOH content>pyrolysis temperature>heating rate?constant temperature time.4.In the catalytic pyrolysis of coal,the change process of functional groups and carbon structure of coal matrix,the diameter distribution,wall parameters and end morphology of carbon nanotubes were as follows:In the process of catalytic pyrolysis,the-CH₃,-CH₂ and other groups in coal were transformed to different degrees;the graphitization degree of catalytic pyrolysis products was higher than that of raw coal,and the ratio of G and D peak strengths of Raman analysis was I_G/I_D=3.0;the product contains a large number of carbon nanotubes with good graphite crystal structure,the number of tube walls was 18-65 layers,the spacing between layers was about 0.34 nm,the diameter was 20-155 nm,and the end of carbon nanotubes was in the shape of opening or in the shape of"nano capsule"or"nano angle"structure.5.In the process of coal catalytic pyrolysis,the potassium catalyst had the double functions of catalyzing the formation of carbon nanotubes and etching the macromolecular structure to form carbon source.The growth of carbon nanotubes was affected by"catalyst catalysis"and"carbon nanotubes self catalysis".The number of graphene layers on the surface of catalyst particles decreases gradually,and the growth process of carbon nanotubes conforms to the"stepwise"growth model.6.The effects of acid pickling and KOH addition on the structure and pyrolysis of coal were as follows:HCl-HF-HCl pickling and demineralization can effectively reduced the mineral content of coal,and had little effect on the aliphatic and aromatic organic carbon structure of raw coal.KOH played an important role in the formation of micropores and the increased of specific surface area and pore volume.During the coal pyrolysis process,the primary Fe minerals in coal migrate from the inside of coal particles to the surface and accumulated in some areas to form nano Fe particles,which are the main catalyst for the growth of carbon nanotubes.7.The growth of carbon nanotubes catalyzed by primary Fe minerals in coal conformed to the mechanism of Fe CO₃??-Fe?Fe₃C+graphite?CNTs.Firstly,the siderite Fe CO₃ embedded in coal was reduced to?-Fe by the aromatic structure carbon of coal macromolecules and the H₂ and CO produced by coal pyrolysis;Secondly,under high temperature,the K component(K₂CO₃,K₂O,K)split CH₄,C₂H₆,etc.and etched the aromatic carbon structure of coal macromolecules to produce carbon atoms or clusters,which were absorbed and integrated into?-Fe and converted it into Fe₃C.The Fe₃C particles exposed by K component etching coal particles fused with each other to form Fe₃C catalyst particles with different particle sizes;Finally,carbon nanotubes were formed on the surface of Fe₃C particles with moderate particle sizes.8.There was significant difference in the concent of carbon nanotubes prepared by KOH catalyted pyrolysis of raw coal and semi-coke.When there was no KOH,the carbon nanotubes didn't appear in the pyrolysis products of raw coal or semi-coke,but after KOH was added,the content of carbon nanotubes in the catalytic pyrolysis products of raw coal or semi-coke gradually increased with the extension of constant temperature time.When the constant temperature time was30 and 90 min,the carbon nanotubes in raw coal+KOH pyrolysis products were6.85 wt%and 10.16 wt%,respectively,and the CNTs in semi-coke+KOH pyrolysis products were 1.07 wt%and 1.67 wt%,respectively.9.The growth mechanism of carbon nanotubes during K-catalyzed pyrolysis of bituminous coal rich in siderite was as follows:the aromatic C-C and C-H structures in coal converted K components such as K₂CO₃ and K₂O into elemental K,and then elemental K converted the carbon atoms in the ether bond C-O-C structure into carbon atoms or carbon clusters.These carbon atoms or carbon clusters generated carbon nanotubes under the action of Fe catalyst particles in coal,and the Fe catalyst came from primary siderite in coal.